Risky fusion power study pays off by bringing plasma close to reactor walls

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The first major fusion collaboration between Chinese and US research teams has released a surprising finding on the future of magnetic confinement fusion: by lowering the distance between the plasma and the wall of the chamber that contains it, they can actually make the system more stable. It could let the researchers create higher-pressure plasmas, and possibly achieve the all-important threshold of ignition, and a self-sustaining fusion reaction.

Magnetic confinement fusion works by using high-energy magnetic fields to, uh, confine a sample of fusion fuel that’s been heated to a plasma state. This can become hotter than the interior of the sun, but they just keep heating it until they force some of the ions in the center to fuse, converting a tiny fraction of their mass to energy, and releasing it. If that plasma were to physically touch the walls of its container, that container would be toast — so it’s important to use those magnetic fields to keep it separated from those walls.

A cross-section of the magnetic confinement rig at ITER.

What this team, from the American General Atomics and China’s ASIPP facility, have found is that by adjusting their use of a principle called bootstrap current, they could let the plasma expand and come closer to the walls of the reaction chamber. At present, heating plasmas will enter a period of instability called “kink mode” in which they oscillate and make it harder to efficiently contain them. By increasing the tokamak’s use of self-generated current (bootstrap current) the researchers found that bringing the plasma closer to the walls could get around kink mode.

It’s a risky decision, to try to bring the heat of a star ever-closer to your multi-billion dollar research rig, but this team did it. You can imagine the level of work-checking that went on, since if their approach to confinement didn’t work, they would almost certainly do some level of damage to the reactor.

This all has to do with the ability of researchers to maintain so-called “magnetic islands” of low plasma turbulence. By using bootstrap current, this study could help scientists to control these islands without the injection of “flow” from outside — which is good, since that’s incredibly difficult and expensive to do at existing magnetic confinement facilities, like the International Thermonuclear Experimental Reactor (ITER). They can create a “pressure-driven” plasma flow that should be easier to control.

A diagram of bootstrap current. Clear now?

The big problem with magnetic confinement fusion is simply that it uses magnets for its confinement — enormous magnetic tokamaks that not only cost an ungodly amount of money, but take decades to produce. A working research reactor would obviously lead to better and more efficiently designed reactors down the road, but without fundamental breakthroughs like high-temperature superconductors they do seem to be many, many years from real-world application.

Right now, ITER and other fusion research facilities are all trying to figure out how to create and confine a fusion reactor efficiently enough to release more energy for capture than they had to inject in the first place. Note that by doing this, and generating the first-ever net joule of fusion energy, would still only produce one net joule of energy. The first successful fusion power generator will be an enormous moment for mankind, but we need modern power plants to output net megajoules, if not gigajoules.

Fusion is making progress toward the technical threshold of power production — but even that momentous achievement will be just the first step of many on the road to powering mankind with the energy of a star.

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